It is often desirable or necessary to leave a motor vehicle unattended with its engine running. Diesel trucks are routinely left running during cold weather because of the difficulty in restarting the engines. Ambulances, fire engines, police cars, and other emergency vehicles are often left running at the site of emergencies to both ensure they are ready to be driven away and to power auxiliary equipment which must continue to run. And there are countless times when delivery vehicles or personal vehicles are left running for short periods of time to avoid wear and tear on the engine and to save gasoline as the operator performs an errand. In these situations, there is a risk that an unauthorized person may attempt to drive the vehicle. Locking the doors to the vehicle provides some security, but is not foolproof. Some vehicles do not have doors (construction equipment, boats, etc.), locked doors can be forced open, and locking the doors serves no purpose when operation of the vehicle by a passenger, such as a child, is to be prevented. Accordingly, there is a need for a security system for motor vehicles which enables them to be left unattended with their engines running, but without the risk of their being driven by an unauthorized person.
A security system for motor vehicles is disclosed in Cardwell, U.S. Pat. No. 4,403,675, issued Sep. 13, 1983. The Cardwell system is activated by engaging the emergency brake and actuating a momentary switch. Once activated, the system bypasses the ignition switch so that the switch can be turned to the "off" position and the key withdrawn without turning off the engine. If the emergency brake is released without the key being first reinserted and the ignition switch turned to the "on" position, the engine turns off. The Cardwell system has apparently failed to achieve commercial success because of two disadvantages. First, the emergency brake must be engaged before the system can be activated. Secondly, and more importantly, the engine of a motor vehicle modified with the Cardwell security system is susceptible to damage if the ignition switch is inadvertently moved past the "on" position to the "start" position, thus engaging the starter into a running engine.
The security system disclosed in Cody, U.S. Pat. No. 4,754,838, issued Jul. 5, 1988, is similar in many respects to the Cardwell system. The Cody system is activated by pushing down and then pulling up on a switch. The system is deactivated by reinserting the key, moving the ignition switch to the "on" position, and then pushing down on the switch. The Cody system is, accordingly, relatively cumbersome to operate and suffers from the same danger as Cardwell in that inadvertently moving the ignition switch to the "start" position can result in serious engine damage.